Back Up Heat Source and Boosting Loop Temps

[stevehull]

Wewant,

Yes, the use of electric resistance strips is common. In fact, all "electric" furnaces are just that - a resistance strip in a non-combustible box.

Please note that I am NOT advocating putting a small electric stove top bread toaster in the return or supply air flex duct!

Most commercial GT heap pumps have space for a resistance heater that is often called "emergency heat". But your electric resistance range oven has exactly the same.

Many people in southern Florida have no home heating units - just AC units. On those few days when it gets cold, just turn on the electric oven, bake some bread, or just leave the oven door open with the baking element turned on. Again note, this is NOT for a gas oven!!

In your area you have far more heating than cooling days. So to keep a home adequately heated, you may need for example a four ton heater (48,000 BTU). But your cooling needs may only be a fraction of this - say 12,000 BTU (1 ton). Most HVAC systems that are one compressor speed will give you four tons of heating or cooling.

If your cooling load is low, as in your area, your AC system will only run for a small part of the time and you end up with cool, "clammy" air in the home in the summer.

This is why a multi-speed compressor is essential that can run on two tons or four tons. Or put in a split system. One example we refer to is the infamous "window shaker" (window AC unit). They cycle on and off, are loud, are inefficient, but are cheap.

AC also is correct to check your gas bill for the last month as you can estimate just how many BTUs it took to keep the house warm in the last month (very cold weather).

But don't think this is long range planning! Get a blower door test done now and then button up. Small amount of $ and some labor can decrease infiltration by easily 50%. This lowers heating cooling bills by 50% right there.

To get back to your original question, it would be silly to preheat the well water by heating it with resistance heating in order to get better efficiency from the GT heat pump. Your water temp is about 40 F and there is a LOT of heat in that water.

A lot of commercial GT heat pumps have a COP of 5 or higher, so think of that.

Have you considered an open loop system? In your area, a lot of people do that and then fill a small pond with water. This lowers your insurance rate (if you don't have local fire hydrants) and you can then irrigate from it in the summer.

You may already be on a well, so all you need is a slightly larger well pump.

Gotta agree with Randen - button up!! You may have lots of insulation, but you gotta check INFILTRATION. A super highly insulated box is worthless when it has a big hole in one end. Get a blower door test.

Steve

[wewantutopia]

My thought of preheating the ground loop would be for "emergency heat" only insead of the resistance heater.

I undersand they have a COP of 5 or higher. If I understand correctly, if you add an additional 1kw to raise the temp of the water you would get 4kw of heat out of the pump. I you use 1kw resistance heater you only get 1kw heat.

Again, this is only me pondering these questions and looking for feedback.

As for my home, I plan on getting a energy audit in the future just out of sheer curiosity. I have already opened and closed the whole thing up so it will be for knowledge only.

Except for the windows/doors I imagine infliltration is pretty low. House wrapped then 1.5" foil faced exterior poly iso with taped seams. Between ALL the studs/framing has been spray foamed with rigid foam.

Anywho, just bouncing ideas off you guys. I love this site!

[AC_Hacker]

Originally Posted by wewantutopia

My thought of preheating the ground loop would be for "emergency heat" only insead of the resistance heater.

I undersand they have a COP of 5 or higher. If I understand correctly, if you add an additional 1kw to raise the temp of the water you would get 4kw of heat out of the pump. I you use 1kw resistance heater you only get 1kw heat.

Again, this is only me pondering these questions and looking for feedback.

As for my home, I plan on getting a energy audit in the future just out of sheer curiosity. I have already opened and closed the whole thing up so it will be for knowledge only.

Except for the windows/doors I imagine infliltration is pretty low. House wrapped then 1.5" foil faced exterior poly iso with taped seams. Between ALL the studs/framing has been spray foamed with rigid foam.

Anywho, just bouncing ideas off you guys. I love this site!

I think you need to do some reading.

Wikipedia has a good section on heat pumps.

Then you will be armed with knowledge, and able to ask the questions you really want to ask.

-AC

[jeff5may]

If you're going to DIY, just convert a window AC unit into a second-stage unit. It's not that difficult with a water source. Just plumb in a plate heat exchanger to replace the indoor air coil. How you handle the air side is up to you. Since it would only be used for heating, there would be no need for a reversing valve or defrost controls. Run in tandem with your "main" (larger) heat pump, it would accomplish your fantasy COP wise.

Considering your high home envelope standard, the extra ton or less of heating would get you through those especially frigid spells. It could just be rigged in "low ambient" mode, to always run under a certain outdoor temperature.

[MN Renovator]

Originally Posted by wewantutopia

Hi everyone, thanks for the responses.

I live in a ~1000 sq. ft. 1910 bungalow that I have insulated to just under passive house standards (the builders grade double pane windows the previous owners installed are the week spot) including R80 attic, 1.5" exterior poly iso, 3" poly iso foamed in between the studs, 1.5" XPS that I hand dug around the basement to below the footings. The walls come to about R30. Done.

I have a prototype solar air heater (really it is version 3.0) and when I rebuild my front porch this spring I will be incorporating 2 of them into the porch build (pictures to follow).

I plan on installing a 10kw solar array in the relatively near future so I'd like to be all electric. I'm sick of paying the gas company for the connection fee (~$30/month) when it is not in use.

I understand the major cost comes from digging the ground loops. I plan on DIY so I'm not concerned.

I've read about using a slightly undersized pump that's why I was thinking back up is needed.

My thinking about the in line tankless just before the heat pump is the heat would be extracted by the pump thus not heating up the ground loop.

Also, I was thinking with the high COP of the heat pump, if you raised the water temp by 1kw you would get the equivalent of 2kw etc. of heat. If you put it after the heat pump you would only get the 1kw worth of heat.

This line of though is purely academic and out of curiosity since the project is many years away.

I've cut the infiltration on my house down to as little as I can outside of what leaks my fairly decent(but still horizontal sliders) window gaskets and the sliding glass door is infiltrating along with a recently discovered air leak at the baseboards of the two cantilevered upstairs bedrooms. I've got the infiltration low enough to where I have enough pressure through those small gaps get frost at EVERY infiltration point in the house. My house insulation is actually decent for 1985 standards but R13 in the walls and 3/4" rigid on the outside. I'm not even R20, my attic has 7" of cellulose(R25ish) and I'm getting 17762BTUhr days when it is -15f outside, 68 inside and cloudy outside all day.

If you've got an R80 attic, R30 walls and you've got insulation 1.5" XPS for the basement in a 1000 sq ft place, I'm not sure you're going to run into an issue with the GSHP being too small unless you have infiltration issues. What's the CFM on your blower door. Do all the air sealing you can, get a blower door test and if you are under 400 CFM at 50 pascals, you could probably get away with a 1 ton GSHP at any temperature as a rough guess. This depends a bit on your glass too though, if you've got a lot that might add something. Since you are going GSHP you have less to worry about with losing output when it gets brutally cold as long as your loop field is big enough. I'd suggest doing what I did, figure out how to do a Manual J load calc and do one. You'd want to be sure you size for your cooling appropriately.

"I can cool the house with a single well placed 12,000 btu window unit and our current furnace is 60,000 btu. How would one comprimise with the GSHP?"

Sounds like you've got your cooling load figured out. Your current furnace was probably put in before you added all of this insulation and probably did the job just fine without even hitting full duty cycle. Have you paid attention to a period of say 6 hours and timed how long it runs on a day that gets about as cold as it ever gets when there is no sun, you've had the chances so far this winter and if you haven't already you've got a good chance to do it now because if you time how long the furnace is running you can get your heat load that way. It is exactly how I know how much energy my house requires to heat with. It's actually less than my Manual J estimate even if I exaggerate my insulation numbers and skip entering infiltration.

What size GSHP were you aiming for prior to creating this thread anyway?

Remember if you are pulling 5kw of heat out of the ground, its because it had 5kw of heat. You were just pumping the 5kw of heat out using 1kw of energy. If you raise the water temp by adding 1kw to it, you'll only get 1kw out of it. You'd be better off using resistance heating inside the house rather than adding it to the loop.

[AC_Hacker]

Originally Posted by MN Renovator

If you raise the water temp by adding 1kw to it, you'll only get 1kw out of it. You'd be better off using resistance heating inside the house rather than adding it to the loop.

When I first read his idea, I visualized that his GSHP would be extracting heat from the outside loop, and would be yielding the heat up to an inside loop that was part of a hydronic floor heating system (which would be a great idea).

In that case, adding electric heat to the inside hydronic loop would be a viable and very sanitary way to add axillary heat.

It would be the hydronic analog to heating coils in an air handler to supply axillary heat.

-AC

[MN Renovator]

right, adding aux heat to an indoor ground loop or if handled through an air handler heating coils would be appropriate in that case too. It sounded the other way around when the OP described it because adding to the outside loop wouldn't work to boost efficiency.

[Blue Bomber Man]

I just wanted to take the time to address one point mentioned earlier.

The idea was brought up to put say a 1 kw electric heat to prewarm the loop temp and was believed by the original poster to allow the heat pump to perhaps get 2 kw heat back from this extra heat in the loop.

This would violate conservation of energy.

To prove my point imagine a hypothetical ground loop with 0 K temp. Lets say you add 1kwh of energy to it and it raises the temp by some amount. You could not then run this through a heat pump (say a COP of 2) and get back 2 kwh of heat, as the system only has 1kwh of heat in it.

So my conclusion is that it would probably be better to directly heat the space with the electric heat, or add it just after the heat pump in your distribution system

[wewantutopia]

This:

"Remember if you are pulling 5kw of heat out of the ground, its because it had 5kw of heat. You were just pumping the 5kw of heat out using 1kw of energy. If you raise the water temp by adding 1kw to it, you'll only get 1kw out of it."

Of course, I don't know how I forgot/overlooked that point!!!!!!

Also: "This would violate conservation of energy." Of course!!

Not sure what I was thinking.

Thanks for knocking some sense back into me!

I haven't thought about what size GSHP I would install yet. It's too far down the road. I was just thinking about concespts. Also, I guess a lot of what I have read/was thinking about is ASHP so as the ambient outside temp drops it will not hold enough heat to extract thus needing emergency heat.

It would be great to only need the heat pump.

I plan on using it to heat radiant heat floors (hardwood with heat spreaders) so if need be, as suggested, I could use a tankless to boost floor temps.

My windows REALLY are the weak point in our home. We have quite a few of them and they are pretty large. One of my son's walls is and exterior wall with almost all windows. Granted, it is south facing with a nice overhang and mature decidious trees, on sunny days in the winter it is great! Not so much at night and cloudy days.

I am waiting for commercially available vacuum insulated windows to replace our curent ones. Ply Gem is expected to start selling the this year... FINALLY!

[NiHaoMike]

If you are on a (rare) "free electricity at night" program, heating the ground loop can be very useful. (Of course, in that case, you'll probably be better off with above ground thermal storage...) Might still be worth something if electricity prices change a lot on and off peak.